Two-dimensional materials integrated with multiferroic layers

ABSTRACT

The invention relates to heterostructures including a layer of a two-dimensional material placed on a multiferroic layer. An ordered array of differing polarization domains in the multiferroic layer produces corresponding domains having differing properties in the two-dimensional material. When the multiferroic layer is ferroelectric, the ferroelectric polarization domains in the layer produce local electric fields that penetrate the two-dimensional material. The local electric fields can influence properties of the two-dimensional material, including carrier density, transport properties, optical properties, surface chemistry, piezoelectric-induced strain, magnetic properties, and interlayer spacing. Methods for producing the heterostructures are provided. Devices incorporating the heterostructures are also provided, including tunable sensors, optical emitters, and programmable logic gates.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application claiming the benefit ofU.S. patent application Ser. No. 15/819,929, filed on Nov. 21, 2017,which claims priority to U.S. Provisional Application No. 62/424,711,filed on Nov. 21, 2016. The entire contents of these applications areincorporated by reference herein.

FEDERALLY-SPONSORED RESEARCH AND DEVELOPMENT

The United States Government has ownership rights in this invention.Licensing inquiries may be directed to Office of Technology Transfer, USNaval Research Laboratory, Code 1004, Washington, DC 20375, USA;+1.202.767.7230; techtran@nrl.navy.mil, referencing NC 104148-US3.

TECHNICAL FIELD

This application relates generally to heterostructures including a layerof a two-dimensional material placed on a multiferroic layer. An orderedarray of differing polarization domains in the multiferroic layerproduces corresponding domains having differing properties in thetwo-dimensional material. When the multiferroic layer is ferroelectric,the ferroelectric polarization domains in the layer produce localelectric fields that penetrate the two-dimensional material. The localelectric fields can influence properties of the two-dimensionalmaterial, including carrier density, transport properties, opticalproperties, surface chemistry, piezoelectric-induced strain, magneticproperties, and interlayer spacing. Methods for producing theheterostructures are provided. Devices incorporating theheterostructures are also provided.

BACKGROUND

The conventional method for introducing an electric field and varyingthe carrier density in a semiconductor channel is to use anelectrostatic gate consisting of a gate dielectric layer and a metalcontact layer over the semiconductor transport channel. Such gates arefabricated using standard lithographic techniques, and they are normallyused to apply an electric field, but their geometry is fixed once theyare fabricated. The intent is to control charge flow, not determine theluminescent or chemical sensing properties. Strain is normallyintroduced by mechanical means.

Ferroelectric films have recently been used as the gate dielectric intransistor structures because their polarization is non-volatile (itremains after the gate voltage has been removed). A ferroelectrictransistor utilizes the nonvolatile, switchable polarization field of aferroelectric gate to control the charge carrier density in theconducting channel. This approach is actively researched as an avenue tononvolatile transistor memory, known as ferroelectric random accessmemory (FeRAM).

However, the conventional methods suffer from several drawbacks. Thelateral size of such a gate is determined by the limits of thelithography used to produce it, and it cannot be changed once it isfabricated. The gate is typically on the top surface, and thereforecovers the active material, preventing its use as either a chemicalvapor sensor or as an optical emitter. The gate requires constant power,and electric field or charge disappears once the power is removed—suchgates are volatile rather than non-volatile. Finally, doping withimpurity atoms is irreversible, and introduces potential defects.

SUMMARY OF THE INVENTION

The invention described herein, including the various aspects and/orembodiments thereof, meets the unmet needs of the art, as well asothers, by providing heterostructures including a layer of atwo-dimensional material placed on a multiferroic layer. An orderedarray of differing polarization domains in the multiferroic layerproduces corresponding domains having differing properties in thetwo-dimensional material. When the multiferroic layer is ferroelectric,the ferroelectric polarization domains in the layer produce localelectric fields that penetrate the two-dimensional material. The localelectric fields can influence properties of the two-dimensionalmaterial, including carrier density, transport properties, opticalproperties, surface chemistry, piezoelectric-induced strain, magneticproperties, and interlayer spacing. Methods for producing theheterostructures are provided. Devices incorporating theheterostructures are also provided.

In accordance with one aspect of the invention, a heterostructureincludes a multiferroic material layer; and a two-dimensional materiallayer provided on the multiferroic material layer. The multiferroicmaterial layer comprises an array of polarization domains in themultiferroic layer, and produces corresponding domains in thetwo-dimensional material.

According to another aspect of the invention, a detector includes aferroelectric material layer; and a two-dimensional material layerprovided on the ferroelectric material layer. The ferroelectric materiallayer includes an array of dipole domains in the ferroelectric materiallayer, and produces corresponding n-type and p-type domains in thetwo-dimensional material layer. In some aspects, the n-type and p-typedomains in the two-dimensional material layer can be used tosimultaneously detect electron-donating chemicals and hole-donatingchemicals within same sensor. The detector of the invention enables morecomprehensive detection and more accurate identification of detectedmolecules.

According to a further aspect of the invention, a method for forming aheterostructure includes providing a multiferroic material layer;applying a local electric field to the multiferroic material layer,forming an array of polarization domains in the multiferroic layer; andproviding a two-dimensional material layer on the multiferroic materiallayer having an array of polarization domains therein, wherein thepolarization domains produce corresponding domains in thetwo-dimensional material.

Other features and advantages of the present invention will becomeapparent to those skilled in the art upon examination of the followingor upon learning by practice of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the relative ΔG/G₀ for an n-type MoS₂monolayer exposed to electron-donating molecules and hole-donatingmolecules.

FIG. 2 is a depiction of a detection device in which polarizationdomains in an underlying ferroelectric layer create n-type and p-typedomains.

FIG. 3(a) depicts a generic mechanical transfer method. FIG. 3(b)depicts a PMMA film transfer method. FIG. 3(c) depicts a PDMS filmtransfer method.

FIG. 4 depicts a configuration for polarization domains, written into a150 nm PZT/Pt/SiO₂/Si test sample using a conductive atomic forcemicroscope (CAFM).

FIG. 5(a) is an image of a 100 nm PZT film surface poled using an atomicforce microscopy (AFM) operated in the electrostatic force microscopyphase mode. FIG. 5(b) is a horizontal EFM line scan averaged left toright across the top two panels of the checkerboard of FIG. 5(a). FIG.5(c) is a schematic cross section of the PZT film, illustrating theorientation of the polarization domains and the corresponding surfacecharge. FIG. 5(d) is a PL peak intensity map obtained from the WS₂monolayer. FIG. 5(e) is a spatial map of the PL linewidth (FWHM)corresponding to the data of FIG. 5(d).

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The invention described herein, including the various aspects and/orembodiments thereof, meets the unmet needs of the art, as well asothers, by providing heterostructures including at least one layer of atwo-dimensional material adjacent to a multiferroic layer. An orderedarray of differing polarization domains in the multiferroic layerproduces corresponding domains having differing properties in thetwo-dimensional material. When the multiferroic layer is ferroelectric,the ferroelectric polarization domains in the layer produce localelectric fields that penetrate the two-dimensional material. The localelectric fields can influence properties of the two-dimensionalmaterial, including carrier density, transport properties, opticalproperties, surface chemistry, piezoelectric-induced strain, magneticproperties, and interlayer spacing. Methods for producing theheterostructures are provided. Devices incorporating theheterostructures are also provided.

The invention provides ways to control and modulate the environmentsurrounding 2D materials on the nanometer length scale by coupling 2Dmaterials with multiferroic materials. In some aspects of the invention,a heterostructure is provided in which one layer is comprised of one ormore 2D materials (either a single monolayer, or multiple monolayershaving the same or different composition), and the adjacent layer iscomprised of a multiferroic material. This multiferroic material can bea ferroelectric material, where local electrostatic domains consistingof dipole ensembles produce a local electric field, modifying thedielectric environment. If the ferroelectric material is a thin film,the strength of the electric field is related to the thickness of thefilm. These domains can be oriented by a global applied electric field,or manipulated at the micron to nanoscale levels with an optical beam,proximal probe such as a conducting atomic force microscope tip (asillustrated in FIGS. 5(a)-5(e)), or other techniques, including opticalprobes.

These polarization domains in the multiferroic or ferroelectric materialcan directly change the properties of adjacent 2D material monolayer(s),which are strongly affected by their immediate environment due to lackof bulk screening. The dielectric screening is very low due to theirtwo-dimensional character relative to bulk material, and the screeningthat would normally occur due to carriers in a three-dimensionalmaterial is largely absent. These changes in environment in turndramatically impact the optical, electronic and mechanical properties ofthe 2D material. Modification of the properties of the 2D material canbe accomplished, for example, by electrostatic and piezoelectricmechanisms, or by changing the layer spacing between the monolayers in amultiple-monolayer 2D material film. Properties of 2D materials andtheir heterostructures can be modified and controlled by variations inthe local electric field induced by local dipoles in an adjacentferroelectric layer. This effect and mechanism is distinct from anelectric field applied by a standard gate terminal.

Heterostructures and Devices.

The heterostructures of the invention include a multiferroic layeradjacent to (and preferably directly in contact with) one or moretwo-dimensional material layers.

The two-dimensional (“2D”) materials of the invention may be used as asingle monolayer, or provided as multiple monolayers. When multiplemonolayers are used, preferably from 2 to 20 monolayers are provided,more preferably from 2 to 10, still more preferably from 3 to 6. Themonolayers may be formed from a variety of materials, includingtransition metal dichalcogenides (“TMDs”), silicene, phosphorene, andgraphene. The TMDs for use in the apparatus and methods of the inventionhave the chemical formula MX₂, where M is a transition metal, and X is achalcogen.

Transition metals include elements from Groups 3-12 of the periodictable. The transition metals include Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu,Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, La, Hf, Ta, W, Re, Os, Ir,Pt, Au, Hg, Ac, Rf, Db, Sg, Bh, Hs, Mt, Ds, Rg, and Cn, as well as thelanthanide series elements (La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho,Er, Tm, Yb, and Lu), and actinide series elements (Ac, Th, Pa, U, Np,Pu, Am, Cm, Bk, Cf, Es, Fm, Md, No, Lr). Preferred transition metals foruse in the apparatus and methods of the invention include Mo, W, Nb, Hf,Ta, and V, with Mo, W, Nb, and Ta being particularly preferred.

Chalcogens include the elements found in Group 16 of the periodic table.The chalcogens include O, S, Se, Te, and Po. Preferred chalcogens foruse in the apparatus and methods of the invention include S, Se, and Te,with S and Se being particularly preferred.

In some aspects of the invention, preferred 2D TMD materials for use inthe sensors, systems, and methods may be selected from the groupconsisting of MoS₂, MoSe₂, WS₂, WSe₂, VS₂, VSe₂, VTe₂, NbS₂, NbSe₂,TaS₂, TaSe₂, and combinations thereof. Additional TMD materials formedfrom the transition metals and chalcogens set forth above are alsowithin the scope of the invention.

The invention also beneficially eliminates the need for impurity dopingto create n- or p-type regions in a 2D TMD layer. TMD monolayers areknown to be ambipolar (i.e., they can be biased to be either n- orp-type). The heterostructures of the invention incorporatingferroelectric materials having arrays of polarization domains enable thecreation of n- and p-type domains within the TMD layer, without the needfor impurity doping, which is very difficult to accomplish in amonolayer.

The multiferroic material can be any material that exhibits more thanone primary ferroic order parameter (which include ferromagnetism,ferroelectricity, and ferroelasticity). In some aspects of theinvention, the multiferroic material is a ferroelectric (FE) material,where local electrostatic domains consisting of dipole ensembles existand produce a local electric field, modifying the dielectricenvironment. These local domains modify and control the optical,transport and other electronic properties of the two-dimensionalmaterial. The multiferroic materials may be selected from BiMnO₃,LaMnO₃, and BiFeO₃, and combinations thereof. Ferroelectric materialsmay be selected from lead zirconate titanate (PZT), barium titanate,lead titanate, lead magnesium niobate-lead titanate (PMN-PT), andcombinations thereof. Composites of any of these materials may also beused.

The multiferroic layer used in the invention may be provided as a bulksubstrate or thin film. When the multiferroic layer is a thin film, itmay optionally be supported by a substrate material such as silica(SiO₂), silicon, SiO₂/Si, or sapphire, but the invention is not limitedto any particular substrate material. For example, growth ofmultiferroic or ferroelectric thin films on periodically poled wafersmay be preferred in some aspects of the invention, to provide laterallytemplated growth of layers deposited thereon. Examples of substratesthat may be provided, optionally as templated substrates, are lithiumniobate and strontium titanate.

The 2D material monolayers of the invention may be exfoliated,mechanically transferred, or grown directly on the multiferroic orferroelectric material. For example, multiple methods are available tomechanically transfer TMD monolayers, such as WS₂, onto alternatesubstrates, such as multiferroic or ferroelectric films. The transfersmay be conducted using a thin layer of a transfer material, such as aPMMA (polymethyl methacrylate), PC (polycarbonate), or PDMS(polydimethylsiloxane)/PC film. The direct integration of themultiferroic with the 2D material results in the smallest possibleseparation (≤1 nm) between the two, thereby maximizing the electricfield resulting from the polarization domains in the ferroelectricmaterial. There is no intervening dielectric layer, as there is in aconventional electrostatic gate contact. Because the ferroelectricdipole electric field decreases with distance R, and corresponds toapproximately R⁻³, the electric field strength at the 2D layer is aslarge as it can be when the 2D material lies directly on top of theferroelectric layer. This electric field penetrates the 2D materiallayer and modifies its properties.

Ferroelectric materials exhibit a spontaneous polarization due tointernal electric dipoles which are coupled to the lattice. Typicalexamples include BaTiO₃, BiFeO₃, and PbTiO₃. They can be polarized in aparticular direction and manner by a global applied electric field—thispolarization is retained even after the electric field is removed (thisis analogous to a magnetic material which exhibits a spontaneousmagnetization, and the magnetization is retained in the absence of anapplied magnetic field). The polarization can also be reversed by aglobal applied electric field, and the hysteresis depends upon factorsthat are both intrinsic (e.g., coupling of the internal dipoles to thelattice) and extrinsic (e.g., interfaces, sample structure and aspectratio).

Ensembles of these dipoles form local domains within the ferroelectricmaterial, with a net polarization oriented in a particular direction (upor down), just as magnetic domains exist in a ferromagnet. The inventionbeneficially provides methods for forming local domains in theheterostructures and devices, which can be oriented and manipulated onlength scales ranging from microns to nanometers by application of ahighly localized electric field applied, for example, using proximalprobe techniques, such as through a voltage applied between theferroelectric material surface and the tip of an atomic force microscope(AFM), which is preferably operated as a conducting atomic forcemicroscope (CAFM). Isolated domains can be created in predeterminedlocations, or an ordered array of domains may be fabricated. Thus, theproperties of the adjacent 2D materials can be controlled and modifiedwith the same spatial resolution, i.e., if a 10 μm×10 μm checkerboardpattern is created in the ferroelectric material, the properties of the2D material layer will also be modified in a 10 μm×10 μm pattern.

The heterostructures of the invention allow for writing and rewritingthe polarization domains of the ferroelectric material in any order,size, spacing, or period, and at any time, in a non-destructive andreversible fashion, permitting the heterostructures to form the basis ofa reconfigurable electronic system. The polarization domains may beprovided in any arrangement, without limitation. Exemplary polarizationdomain configurations include a checkerboard pattern, or concentricshapes (including, without limitation, squares, rectangles, circles,ovals, shapes exhibiting one or more axes of symmetry, or irregularshapes). The polarization domains are non-volatile, and no refresh poweris required. The heterostructures also permit a global erase function,which may be achieved when a global electric field is used to erase anydomains written in the ferroelectric layer.

The heterostructures of the invention may be used in a device that iscapable of operating based on modifications in carrier density,transport properties, optical properties, surface chemistry,piezoelectric-induced strain, magnetic properties, and/or interlayerspacing. The heterostructures of the invention are broadly applicable tothe field of low-power electronics (e.g., for distributed autonomoussystems), detectors (e.g., for chemical vapor sensors for biologicalagent and explosives detection, including, but not limited to, TEA(triethylamine), THF (tetrahydrofuran), DCB (dichlorobenzene), MeOH(methanol), NT (nitrotoluene), and DCP (dichloropentane)), transistors(including radio-frequency transistors), frequency-agile electronic andoptical devices, diodes, electroluminescent/light-emitting devices,structures exhibiting negative differential resistance, programmablelogic gates, and non-volatile transistor memory. The multiferroicstructures of the invention may also be used for transduction and fordetection of magnetic fields. The development of hybrid 2D/multiferroicheterostructures further provides new avenues for spin electronics(“spintronics”).

When used in devices, the heterostructures of the invention mayoptionally be combined with any suitable components, including, but notlimited to, electronic contacts, and electromagnetic signaltransmitters. Signal transmitters may optionally be used, for example,to generate a signal to indicate that the 2D material layer hasinteracted with an agent of interest. However, it is to be appreciatedthat one of the benefits of the invention is the simplified fabricationthat it permits by eliminating deposition and lithography stepsnecessary to deposit and define dielectric layers and top metal layers,as no discrete insulating dielectric layer or top metal contact arerequired to introduce the local electric fields. In addition, theinvention offers lateral spatial resolution that is comparable to orbetter than that available with existing lithographic techniques.

In one preferred application for the heterostructures of the invention,sensors are provided. The conductivity of monolayer TMDs, such as n-typeMoS₂ monolayers, increases when exposed to electron-donating molecules(labeled “e”), and shows no response to hole-donating molecules (labeled“h”). See FIG. 1 . The sensor devices of the invention use thepolarization domains in ferroelectric materials underlying the TMDmonolayer(s) to create local n- and p-type domains in the TMDmonolayer(s), which beneficially permits simultaneous detection ofelectron- and hole-donating molecules with same detector, enabling morecomprehensive detection and more positive identification. See FIG. 2 .

Modifying Properties of 2D Materials.

In accordance with another aspect of the invention, the local electricfield provided by dipoles in the ferroelectric material layer modifiesthe carrier density in the 2D monolayer(s), which strongly affectstransport properties, mechanical properties, optical properties(emission or absorption), as well as surface chemistry. An ordered arrayof polarization domains of the ferroelectric material produces acorresponding ordered variation in carrier densities of the 2D material.A periodic array of such domains may impose a super period on the 2Dlayer, and a corresponding change in band structure. Lower carrierdensities lead to formation of excitons, while higher carrier densitieslead to formation of charged excitons, called trions. The relativepopulations of excitons and trions determines the optical emissionspectrum of the 2D material. In this way, the heterostructures of theinvention may be configured for light emission or absorption.Accordingly, devices including photo luminescent and light-emittingdiodes are provided by the invention.

Varying the carrier density also impacts the way in which the surfaceinteracts with molecules in the gas phase, and therefore controls theselectivity and responsivity of the 2D material as a chemical sensor,such as a chemical vapor sensor. The resistivity of single or fewmonolayer materials changes with charge transfer from surfaceadsorbates, and detection of changes in resistivity can be useful forsensing applications, such as chemical sensing. The change inresistivity with exposure to a given molecule depends upon whether thematerial is n- or p-type. The polarization domains in an adjacentferroelectric layer therefore can control these properties. Theseproperties may also be used in applications such as low powerelectronics, chemical vapor sensors, and optical devices.

In other aspects of the invention, when the 2D material layer ispiezoelectric, the polarization domains in the adjacent ferroelectriclayer create local strain domains in the 2D material with a spatialdistribution that is directly determined by and correlated with thepolarization domains. An ordered array of polarization domains producesa corresponding ordered variation in strain fields. The optical andelectronic properties of the 2D materials are strongly affected bystrain. Small changes in lattice constant can change the 2D layer from adirect gap to an indirect gap semiconductor, or change a non-magneticmaterial to a magnetic material. This aspect of the invention providescontrol of the magnetization, magnetic domain structure, and switchingby controlling the polarization domain structure in the ferroelectriclayer with an electric field. The heterostructures of the invention alsoprovide a means to introduce strain in piezoelectric 2D materials at thenanoscale without resorting to mechanical fixtures.

In further aspects of the invention, monolayers of 2D materials may beattracted to one another by the van der Waals interaction to formbilayers, multilayered films, and bulk materials. Each monolayer can bethe same material, or consist of different materials to form a novel vander Waals heterostructure. The spacing between the monolayers dependsupon the strength of the van der Waals interaction, and this spacingdetermines the band structure and corresponding electronic and opticalproperties. The van der Waals interaction is the force which exists dueto interactions between dipoles. Thus by placing a 2D material on aferroelectric layer, the dipoles in the ferroelectric layer alter thebinding force between adjacent monolayers of the 2D materials, changethe layer spacing, and alter the optical and electronic properties. Ifan array of dipole domains is patterned in the ferroelectric layer, thenthe properties of the 2D material exhibit a corresponding spatialdependence. The heterostructures of the invention beneficially offer away to alter, control, and even modulate the spacing between twomonolayers of 2D materials. This spacing is a significant factor indetermining the optical and electronic properties of the system, and isnormally determined by the natural van der Waals interaction.

Methods.

The invention also provides methods for forming heterostructures,including, but not limited to, the heterostructures described herein.The methods include providing a multiferroic material layer and applyinga local electric field to the multiferroic material layer in order tocreate one or more polarization domains in the multiferroic layer. Forexample, when the multiferroic material layer is a ferroelectricmaterial layer, the polarization domains may comprise dipole domains.

The local electric field may be applied, for example, using an opticalbeam, a proximal probe (such as a conducting atomic force microscopetip), or other techniques including optical probes. When a conductingatomic force microscope is used, it may be operated at a bias voltage offrom ±1 V to ±10 V. When the multiferroic material layer is aferroelectric material, a positive tip voltage will result inpolarization dipoles in the ferroelectric layer that point into thesample plane, and a negative charge at the surface of the ferroelectriclayer. A negative tip voltage will result in polarization dipoles in theferroelectric layer that point out of the sample plane, and a positivecharge at the surface of the ferroelectric layer. An image of the poledsurface may also be obtained using the atomic force microscope byoperating it in EFM phase mode.

The polarization domains may be provided in any size, shape, pattern, orconfiguration that is desired, based on the properties or functions ofthe specific heterostructure being formed. The polarization domains mayrange from a nanometer scale (i.e., features having a width on the orderof 1 nm or more) to multiple micron scale (i.e., features having a widthon the order of 1 micron, 5 microns, 10 microns, or more). Polarizationdomains may be separated by domain walls having any desired width. Insome aspects of the invention, the polarization domain wall width may beas low as from 1-10 nm, though wider domain walls are also included inthe scope of the invention.

In some aspects of the invention, the local electric fields may beglobally erased, for example, by exposing the entire multiferroicmaterial layer, or the entire heterostructure, to a global electricfield. Once erased, the multiferroic material layer may have new localelectric fields applied. The process of globally erasing thepolarization domains and providing a new configuration of polarizationdomains may be repeated multiple times. In other aspects of theinvention, the polarization domains may be modified only in desiredlocations, by applying appropriate local electric fields to areas havingpolarization domains to be changed.

The local electric field may be applied to the multiferroic materiallayer prior to depositing a 2D material layer thereon, or it may beapplied after the heterostructure including the multiferroic layer and2D material layer has been formed. Regardless of the order of thesesteps, the polarization domains in the multiferroic material layerproduce corresponding domains in the two-dimensional material layer thatis provided on the multiferroic material layer. The term “correspondingdomains” is used to refer to domains in a 2D material layer that is partof a heterostructure, where the domains have properties (such as thosedescribed above) that are influenced by or result from proximity to apolarization domain of a multiferroic material. These domains aretypically positioned opposite to a polarization domain formed in amultiferroic material.

The 2D material layer may be applied to the multiferroic material layerusing a technique selected from the group consisting of mechanicalexfoliation, mechanical transfer, and growth directly on themultiferroic material layer.

The 2D material layer may be applied directly to a multiferroic materiallayer in some aspects of the invention. When the 2D material layer isapplied to a substrate and is transferred to the multiferroic layer, thetransfer may be carried out using mechanical techniques. Regardless ofthe material to which it is applied, the 2D material layer may bedeposited by chemical vapor deposition or other deposition or growthtechnique to a thickness of 1 monolayer (which is about 0.7 nm thick fora TMD monolayer, but those skilled in the art will appreciate that thethickness of the monolayer will depend on the specific monolayercomposition). More than one monolayer may also be applied to form the 2Dmaterial layer, either by sequential application of layers or bydepositing multiple layers simultaneously.

EXAMPLES

Aspects of the invention will now be particularly described by way ofexample. However, it will be apparent to one skilled in the art that thespecific details are not required in order to practice the invention.The following descriptions of specific embodiments of the presentinvention are presented for purposes of illustration and description.They are not intended to be exhaustive of or to limit the invention tothe precise forms disclosed. Many modifications and variations arepossible in view of the above teachings. The embodiments are shown anddescribed in order to best explain the principles of the invention andits practical applications, to thereby enable others skilled in the artto best utilize the invention and various embodiments with variousmodifications as are suited to the particular use contemplated.

Example 1

One method for mechanically transferring a TMD monolayer, such as WS₂,onto a substrate, such as a ferroelectric film, includes the use of aPMMA film, as illustrated in FIG. 3(b). A sample including a layer ofWS₂ on an SiO₂ substrate is coated with a thin layer of PMMA (polymethylmethacrylate) resist and cured on a hot plate at 100° C. for 10 minutes,then submerged in buffered hydrofluoric acid to etch the SiO₂, freeingthe WS₂ from the growth substrate. Once fully etched, the film wasrinsed in deionized H₂O, where it floated on the surface, and was thenlifted from the water using the desired substrate. Optionally, adhesionof the WS₂ layer may be improved by spinning at 2000 rpm and baking at100° C. An acetone and isopropanol soak removes the PMMA. An opticalimage of PMMA transferred WS₂ exhibits a uniform, clean, triangularshape, and is also shown in FIG. 3(b).

Example 2

Another method for mechanically transferring a TMD monolayer, such asWS₂, onto a substrate, such as a ferroelectric film, includes the use ofa PDMS/PC film, as illustrated in FIG. 3(c). A sample including a layerof WS₂ on an SiO₂ substrate is brought into contact a PDMS/PC film, thenretracted. This moves the WS₂ from Si/SiO₂ onto the PDMS/PC film. ThePDMS/PC/WS₂ stack is then placed onto clean Si/SiO₂. The PDMS stamp isretracted, leaving the PC film on the top surface of WS₂, which is thendissolved in chloroform. An optical image following PDMS transfer isshown in FIG. 3(c).

Example 3

Polarization domains were written into a 150 nm PZT/Pt/SiO₂/Si testsample using a conductive atomic force microscope (CAFM) manufactured byPark Systems (Suwon, South Korea), in order to demonstrate thatpolarization domains in the ferroelectric film control photoluminescence(PL) intensity of mechanically transferred WS₂ monolayers. The sample isshown in FIG. 4 .

The 6×6 micron image shown in FIG. 4 was obtained using the AFMoperating in electric force microscopy imaging mode. The dark regions ofthe image correspond to the areas of the sample in which the AFM createddipole domains pointing up. The light regions of the image correspond tothe areas of the sample in which the AFM created dipole domains pointingdown. This is shown schematically in the accompanying cross sectioncorresponding to the fiducial line drawn through the image. While thedipole domains shown here are about 500 nm in lateral dimension, domainson the scale of a few nanometers can also be successfully created andimaged.

Example 4

A large area monolayer WS₂ grown by a CVD process on a SiO₂/Si substratein a 2 inch tube furnace. WO₃ powder and sulfur precursors were heatedto 825° C. under a 100 sccm argon and 10 sccm hydrogen flow.Perylene-3,4,9,10-tetracarboxylic acid tetrapotassium salt was used asseed molecules to promote lateral growth. The monolayer nature wasconfirmed by Raman and PL mapping.

The WS₂ film was removed from the SiO₂/Si growth substrate andtransferred onto a 100 nm thick PZT film on a conducting n-typestrontium titanate wafer. Transfer was conducted using the method ofExample 1. Before transfer, a metal marker grid pattern (Ti/Au) wasdeposited on the PZT film using either a shadow mask or photolithographytechnique, in order to assist in locating specific poled areas.

Polarization domains were written into the PZT film using a C-AFM (ParkSystems NX-10), which was operated using dc voltages of up to ±10 V,using two types of cantilevers: Cr—Pt coated (Multi75E, Budget Sensors)and Au-coated (PPP-NCSTAu, Nanosensors) Si cantilevers. Similar resultswere obtained with both. A tip voltage of ±10 V direct current (dc) wasapplied in the contact mode, and polarization domains were written in acheckerboard pattern. Line scan densities of at least 512 lines per 10μm were used to write the polarization domains into the PZT in acheckerboard pattern with the tip polarities shown. Dynamic contactelectrostatic force microscopy was used to image the polarization stateof the poled regions, at a frequency of 75-160 kHz. The total image sizeis 30×30 μm, and each poled square is 10×10 μm. The dashed lines areprovided as a guide, and the bias voltages applied to the C-AFM tip areindicated.

An image of the poled 100 nm PZT surface was obtained using the same AFMoperated in the electrostatic force microscopy phase mode, and an EFMphase image of the area is shown in FIG. 5(a).

FIG. 5(b) shows a horizontal EFM line scan averaged left to right acrossthe top two panels of the checkerboard pattern. There is strong contrastbetween the squares written with opposite AFM tip polarities, indicatingsuccessful poling of the PZT film. There is little contrast between theareas of PZT that were not poled by the AFM and the squares that wereintentionally poled using a +10 V tip bias, due to global poling of theentire PZT film before application of the AFM. FIG. 5(c) shows aschematic cross section of the PZT film illustrating the orientation ofthe polarization domains and corresponding surface charge.

A PL peak intensity map was obtained from the WS₂ monolayer over a 30×30μm area in the sample plane, acquired from a region of the PZT that wasintentionally poled by the AFM with the checkerboard pattern, as shownin FIG. 5(d). A spatial map of the PL linewidth (FWHM) corresponding tothe data of FIG. 5(d) is shown in FIG. 5(e).

It will, of course, be appreciated that the above description has beengiven by way of example only and that modifications in detail may bemade within the scope of the present invention.

Throughout this application, various patents and publications have beencited. The disclosures of these patents and publications in theirentireties are hereby incorporated by reference into this application,in order to more fully describe the state of the art to which thisinvention pertains.

The invention is capable of modification, alteration, and equivalents inform and function, as will occur to those ordinarily skilled in thepertinent arts having the benefit of this disclosure. While the presentinvention has been described with respect to what are presentlyconsidered the preferred embodiments, the invention is not so limited.To the contrary, the invention is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the description provided above.

What is claimed:
 1. A heterostructure comprising: a multiferroicmaterial layer; a two-dimensional material layer provided adjacent tothe multiferroic material layer; a first terminal contacting themultiferroic material layer and a first portion of the two-dimensionalmaterial layer; a second terminal contacting the multiferroic materiallayer, and the first portion of the two-dimensional material layer, anda second portion of the two-dimensional material layer; and a thirdterminal contacting the multiferroic material layer and the secondportion of the two-dimensional material layer; wherein the multiferroicmaterial layer comprises an array of polarization domains in themultiferroic layer, and produces corresponding domains in thetwo-dimensional material; and wherein the local electric fields createlocal n-type and p-type domains simultaneously in the two-dimensionalmaterial layer.
 2. The heterostructure of claim 1, wherein the p-typeand n-type domains can be switched; wherein the switching of the p-typeand n-type domains changes the in-plane conductivity; wherein the secondterminal simultaneously measures a change in resistance of the n-typeand p-type domains as they are exposed to a particular analyte; andwherein the second terminal switches the polarization of themultiferroelectric material layer and the two-dimensional materiallayer.
 3. The heterostructure of claim 2, wherein the multiferroicmaterial layer is a ferroelectric material layer; wherein theferroelectric polarization domains in the ferroelectric material layerproduce local electric fields that penetrate the two-dimensionalmaterial layer; and wherein the ferroelectric material layer comprisesan array of dipole domains in the ferroelectric material layer.
 4. Theheterostructure of claim 3, wherein the local electric fields modifyproperties of the corresponding domains in the two-dimensional material;and wherein the multiferroic material layer is selected from the groupconsisting of BiMnO₃, LaMnO₃, and BiFeO₃ and combinations thereof. 5.The heterostructure of claim 4, wherein the properties are selected fromthe group consisting of carrier density, transport properties, opticalproperties, surface chemistry, piezoelectric-induced strain, magneticproperties, and interlayer spacing.
 6. The heterostructure of claim 2,wherein the ferroelectric material layer is selected from the groupconsisting of lead zirconate titanate (PZT), barium titanate, leadtitanate, lead magnesium niobate-lead titanate (PMN-PT), andcombinations thereof.
 7. The heterostructure of claim 1, wherein thetwo-dimensional material layer is selected from the group consisting oftransition metal dichalcogenide (TMD), silicene, phosphorene, graphene,and combinations thereof.
 8. The heterostructure of claim 7, wherein thetwo-dimensional material layer is a TMD selected from the groupconsisting of MoS₂, MoSe₂, WS₂, and WSe₂.
 9. A device comprising theheterostructure of claim
 1. 10. The device of claim 9, wherein thedevice is selected from the group consisting of sensors, opticalemitters, and programmable logic gates.